Delicate assemblies can be performed even in inaccessible joint locations. Suitable methods are available for mass production.

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Brazing-stainless Steel Base Metals

It should be remarked that Stainless Steel is not a well defined material. This general name indicates all those iron base materials containing at least 10% Cr. The five main classes are briefly described in our page on Stainless Steel Welding.

Duplex types need special attention to avoid embrittlement. BNi-7 was used successfully in hydrogen atmosphere.

The chromium content that gives to stainless steels their most important properties, mainly protection from rusting, produces on the surface a chromium oxide film that must be removed before wetting (by the molten filler metal) and Brazing-stainless can occur.

The same applies to titanium oxide that forms in stainless steel titanium stabilized grades (321). These oxides are tenacious and refractory, and must be prevented from forming again while heating, either by a suitable flux or by the presence of adequately protective atmosphere.

Once a brazing process has been developed with successful results, rigid adherence to process control measures must be assured.

Filler Metals

The selection of suitable filler metal for Brazing-stainless is based on the mechanical properties of the joints, on their corrosion resistance, on the service conditions including operating temperature, stresses and environment, on heat treatment requirements, on the Brazing-stainless temperature and on the heating method.

Furthermore joint geometry and thickness, brazing process, production required per unit time, cost, provisions against sensitization of unstabilized alloys and safety considerations must be taken into account.

Many types are available based on different material families. Most of them are included in

Additional proprietary filler metals may be found in suppliers' catalogs.Silver base Brazing-stainless filler metals are among the most used.
Alloys containing Zn or Cd get off noxious fumes that should not be inhaled by operators. The same are not suitable for furnace brazing.

If the Brazing-stainless temperature is very near to that of sensitization (See Stainless Steel Welding), there is the risk to impair corrosion resistance of the stainless steel involved unless the time at temperature is very short. In case of concern either the base metal or the filler would better be changed.

Additional filler metals were developed for use in vacuum service conditions.

The following Table lists some of the important Silver Base Filler Metals.

Table Silver Base Brazing-stainless Filler Metals

AWS A5.8

Composition %

Temperature C/F

Class

Ag

Cu

Zn

Cd

Ni

Other

Solidus

Liquidus

BAg-1

45

15

16

24

-

-

605/1125

620/1145

BAg-1a

50

15.5

16.5

18

-

-

625/1160

635/1175

BAg-2

35

26

21

18

-

-

605/1125

700/1295

BAg-2a

30

27

23

20

-

-

605/1125

710/1310

BAg-3

50

15.5

15.5

16

3

-

630/1170

690/1270

BAg-4

40

30

28

-

2

-

670/1240

780/1435

BAg-5

45

30

25

-

-

-

675/1250

745/1370

BAg-6

50

34

16

-

-

-

690/1270

775/1425

BAg-7

56

22

17

-

-

Sn=5

620/1145/

650/1205

BAg-8

72

28

-

-

-

-

780/1435

780/1435

BAg-8a

71.7

28

-

-

-

Li=0.3

765/1410

765/1410

BAg-9

65

20

15

-

-

-

670/1240

720/1325

BAg-10

70

20

10

-

-

-

690/1275

740/1360

BAg-11

75

22

3

-

-

-

740/1365

790/1450

BAg-13

54

40

5

-

1

-

720/1325

855/1575

BAg-13a

56

42

-

-

2

-

770/1420

895/1640

BAg-18

60

30

-

-

-

Sn=10

1115/600

1325/720

BAg-19

92.5

Rem

-

-

-

Li=0.2

760/1400

890/1635

BAg-20

30

38

32

-

-

-

675/1250

765/1410

BAg-21

63

28.5

-

-

2.5

Sn=6

690/1275

800/1475

BAg-22

49

16

23

-

4.5

Mn=7.5

680/1260

700/1290

BAg-23

85

-

-

-

-

Mn=15

960/1760

970/1780

BAg-24

50

20

28

-

2

-

660/1220

705/1305

BAg-25

20

40

35

-

-

Mn=5

740/1360

760/1455

BAg-26

25

38

33

-

2

Mn=2

705/1305

800/1475

BAg-27

25

35

26.5

13.5

-

-

605/1125

745/1375

BAg-28

40

30

28

-

-

Sn=2

650/1200

710/1310

BAg-33

25

30

27.5

17.5

-

-

640/1180

715/1320

BAg-34

38

32

28

-

-

Sn=2

650/1200

720/1330

BAg-35

35

32

33

17.5

-

-

685/1265

755/1390

BAg-36

45

27

25

-

-

Sn=3

640/1185

680/1260

BAg-37

25

41

32

-

-

Sn=2

690/1270

780/1435

Notes:

BAg-1 and

BAg-1a: General purpose brazing alloys for thin clearance joints. All metals. Not for corrosion resistance.

BAg-2 and

BAg-2a: Economic general purpose low temperature brazing for variable gaps. Higher brazing temp. Not to be used for prolonged heating time because of liquation.

Nickel Base Brazing-stainless filler metals, suitable for higher service temperatures, are also quite frequently used, generally in furnace brazing. These materials are generally supplied in the form of powder or paste (powder with binder). Paste containers are generally kept under refrigeration before use, because the binder has an expiry date beyond which the paste should not be used.

Additional filler metals based on copper, gold and precious metals, and cobalt are also in use for special applications.

Fluxes

Fluxes are necessary for Brazing-stainless steels in air, by torch or by induction. Manufacturers' recommendations as to their temperature of activity should be followed, depending on the filler metal used.

The protective atmospheres used for Furnace Brazing-stainless steels are, besides vacuum, dry hydrogen and dissociated ammonia. The dew point should be under control and kept low for best results.

Occasionally an inert atmosphere of dry argon can be used if hydrogen is prohibited because of its reactivity to some metals.

Quality

The usual quality requirements are applied, depending on the application. Once the Brazing-stainless Procedure Specification has been approved, process control must be applied in all stages of production from preparation to final delivery.

Safety

Safety should always be a concern, but especially for the danger of inhalation of zinc and cadmium by operators in torch brazing, and for the danger of explosion if hydrogen atmosphere is used. In this case the furnace must have facilities for suitable management of emergencies like sudden loss of power. Training and refreshing of instructions should be repeated at regular intervals.

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